Thermionic relay circuit



April 28, 1942. R. M. KALB 2,281,040*

THERMIONIC RELAY CIRCUIT I Filed April 26, V1941 2 Sheets-Sheet lATTORNEY April 28, 1942. R. M. KALB 2,281,040

THERMIONIC vRELAY CIRCUIT Filed April 26, 1941 2 Sheets-*Sheet 2 HIGHESTNEGATIVE HMS/NG POTENTIAL /NVEA/TOA l?. M. KALB Patented pr. 28, 1942THERMIONIC RELAY CIRCUIT Robert M. Kalb, Madison, N. J., assignor toBell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application April 26, 1941,Serial No. 390,464 1o claims. (ci. 17a-320i This invention relates tothermionic relay circuits and has for its object the production ofcertain useful relay effects.

One object of the invention is to provide means whereby a relay willboth operate and release on the same value of input current whereby anaccurate signal may be given whenever the current passes agiven point ineither direction. Such a signal may be particularly useful in certainrelay voice controlled circuitsvwhere echo Suppressors and likeapparatus are employed.

Another object of the invention is to provide means whereby a relay willoperate and remain operated over a -definite time period in response toan input signal of shorter duration, which may vary over a wide range.

Still another object of the invention is to provide means whereby arelay will operate and remain operated thereafter over an indefinitetime interval in response to short and weak disturbances.

In ac cordance with these objects a thermionic tube is operated in aunilateral feedback circuit and a relay in the output circuit thereof iscontrolled in various manners by the signals applied to the input. Theinvention is an improvement of the thermionic relay circuit disclosed inPatent 2,070,900, granted to Lionel Herbert Harris on February 16, 1937.

A feature of the invention is the use of a multigrid tube operating in afeedback circuit having a comparatively high impedance to current in onedirection and comparatively low impedance to current in the otherdirection whereby economical operation and enhanced effects may besecured even where the tube is operated under. those conditions whereunstable behavior might be expected.

Generally speaking; two kinds of instability are manifest ina feedbackrectifier circuit. One kind is the ambiguity of the plate current whichde pends in its value upon the direction of its change; the other kindconsists of oscillations in the circuit. Heretofore instability has beenavoided and removed by means which has removed the sensitiveness of thevdevice while the present invention aims rather to control and use toadvantage the means and the conditions which lead to unstable behavior.Accordingly, a feature of the invention is a means for securing thebenefit of the phenomena exhibited during unstable behavior whileholding the circuit firmly under control.

Another feature of the invention is a feedback circuit where the platecurrent when raised by an input signal is limited by grid bias potentialand load resistance rather than by the iiow of grid current. Heretoforethe phenomenon of feedback enhancement of the plate current in a triodewas wasteful and uneconomic in that the lifeof the tube was shortened bythe use of grid current as a limiting factor or a limiting means. Thepresent invention employs other factors for limiting purposes andthereby achieves more acceptable operating practices and economicalresuits.

Another feature of the invention is the use of a multigrid tube wherebyoperation of the tube may be achieved in regions where automaticcompensation for changes is achieved. The characteristics of the circuitemploying such a tube are such that values of the various factors in thecircuit may be chosen so as to work in a very stable region. By way ofexample, the resistance of the load may be so chosen that thecharacteristic relation between plate current and grid potentialexhibits a limiting function. The rectified value of the alternatingcurrent components of the plate current show a maximum at an averagevalue of grid potential where there can be noharmful fiow of gridcurrent and a deviation from such maximum on'either side thereof isinhibited by the circuit reaction. Again the gridbiasing potential maybe so chosen that the characteristic relation between plate current andinput to the circuit exhibits a similar limiting function. Here againthe rectified value of the alternating components of the plate currentshow a maximum at a point where there can be no harmful flow of gridcurrent and where a deviation from such maximum is inhibited by thecircuit reaction.

Another feature of the invention is a rectified reaction thermionic tubecircuit comprising a multigrid tube .and a condenser in the feedbackcircuit thereof which exercises a critical control over thecharacteristic relation between the anode current and the input. In somecases the characteristic curve for increasing inputs lies above thecorresponding curve for decreasing inputs and in other cases thisrelation is reversed. In still other instances, the curve for decreasinginputs becomes a straight line since after the plate current hasattained a given value on an increasing input it remains at that valuethereafter regardless of any further change of input in eitherdirection. In this latter case the circuit may be employed in the mannerof a gas tube which when fired maintains a steady flow of current evenafter the activating impulse has ceased. In this manner an ordinaryrelay may be employed to give the same type of operation asa so-calledtrigger tube.

The term rectified reaction thermionic tube circuit used herein will beunderstood to define a thermionic tube circuit having a feedbackconnection between the grid and plate circuits thereof includingalternating current rectifying means for producing a controlling gridpotential by rectifying alternating current in the plate circuitproduced by an original alternating current potential on the gridthereof.

The drawings nine figures.

Figs. 1 to 4, inclusive,rare circuit diagrams illustrating theinvention. Fig. 1 shows the essential elements of the rectified reactionthermionic tube circuit; Fig. 2 shows this same circuit connected to ameans for varying the input for operating the circuit; Fig. 3 -shows howthe feedback circuit may be temporarily disconnected in order to gatherdata for plotting the curves of Fig. 8 for the purpose of arriving at anunderstanding of the unstable behavior of the circuit; and Fig. 4 showsan alternative arrangement of the circuit of Fig. 1 connected to a meansfor stimulating the circuit by impulses; Fig. 5 is a family of curvesshowing the relation between plate current and grid biasing potentialfor different load resistances;

Fig. 6 is a characteristic curve vshowing the consist of two sheetscontaining diierence in the relation of plate current to input underincreasing and decreasing values of input;

Fig. '7 is a family of curves showing the relation of plate current toinput under varying negative grid biasing potentials; l

Fig. 8 is a family of curves showingl the relation between the rectifiedvoltage and the grid potential for various values of input andexplaining the phenomenon of separation of the characteristic curves ofFig. 6 on increasing and decreasing inputs; and

Fig. 9 is a graph showing the hang-over operation of the relay andindicating the peculiar operation thereof when the stimulating impulseis of very short duration.

The vaction of the feedback rectifier can be understood in itsfundamental aspects by reference to the circuit diagram of Fig. 1. Analternating voltage applied to the input terminals I and 2, appears,amplified, on the anode of tube 3. Blocked by the inductance L of thewinding of relay Lan alternating current flows through condenser 5 andthence to ground, through condenser 6 while in one direction anddirectly in the other, as determined bythe two rectifier units 1 and 8.The rectified voltage established across the condenser 6 as a resultbiases the control grid of tube 3 more positive by its average value.

and in the appended claims cuit reduces thepotential'on the plate andthis in turn affects the point at which grid current begins to flow.

The choice of tube is governed by the relay current and the sensitivity.Since the relay current permits some leeway and may be adjusted some bythe tube potentials, the sensitivity becomes the chief factor forconsideration. Sensitivity is the rate of change of the direct currentplate current with respect to the alternating current input voltage.

d10 Io dV dEoV:

where Io represents the plate current; Eis the peak value of thealternating current input voltage and-Vis the vdirect current voltageacross condenser 6. i

If the rectification is linear, this voltage is proportional to thealternating component of the plate current:

where ,t is the amplification factor of the tube, Ro is the plate tofilament resistance,

L 1 X1-wC and X-2wC C1 is the capacity of condenser 5 and C is thecapacity of condenser 6. l The reactances X1 and X are evaluated at thefundamental'frequency. The factor 2 in X results from the half- Pimagm-ov 1t. the sensitivity is thus expressed as follows:

TFRQTX' $11 (3) This relation is rigorous to the extent that the" linearamplification implied by Equation 2 is realized. A rectification that isother than linear vor the flow of grid current or a variable The anodecurrent through the inductance L and resistance R of the winding ofrelay 4 is thereby increased and results in the operation of relay 4.The anode voltage decreases by the drop across the resistance of therelay winding until the diminished gain eiects equilibrium at theultimate anode current. The screen electrode tends to keep the potentialof the space between the control grid and the anode more nearly constantand hence this process is more satisfactory than limiting by the flow ofgrid current, such as is depended upon in triodes. In 'the triodecircuits the load in the anode cirmutual conductance will alter thequantitative results somewhat, but not the conclusions. e

With the reactances fixed by the exigencies of speedy operation thealternating and direct components of the plate current, a high mutualconductance yields the best sensitivity. This is greatest and equalsXgm", when the plate. resistance well exceeds the series reactanceX-l-Xi; it is by the factor Ro X X, when this factoris much less thanunity.

stances allow is a high figure of merit, defined as gmRo. The multigridtube 3 diagrammatically illustrated represents any one of a number ofcommercially available tubes having such a circuit. Therefore, Fig. 2may be used for l.this

purpose. Here an inductance 9 tuned to the` frel quency of the source ofalternating current I0 by the condenser Il is used as a direct current.

path for the application of a direct current poand effectual separationof.r

smallest, reduced Then the criterion for the best sensitivity thecircumtential to the grid of the tube 3 in place of the resistance l2illustrated in Fig. l.

It is desirable as hereinbefore pointed out to regulate this circuit bymeans other than the Vflow of grid current for it is well known thatsuch flow of grid current shortens the life of a tube. This aim isattained in the present invention by taking advantage of certaincharacteristics of the tube, illustrated in Fig. 5. Here is shown ,afamily of curves showing the relation between the current flow in theplate or anode of the tube and the grid potential. Each graph representsa different valueof resistance in the winding of relay 4, and comparisonof the various graphs from a low resistance winding to a high resistancewinding shows a peculiar characteristie which is used to advantageherein. Thus it shows that with the highest reslstance'wlnding a certainplate current is reached which remains at the same level over a widerange of grid potential extending from a minus quantity through zero toa plus quantity. This means that, since if the grid potential is variedperiodically about an average value, fixed by a biasing battery forinstance, the resulting -alternating component in the plate current willhave that one of a wide range of amplitudes which is determined by theaverage grid potential that is maintained, that, amplitude will be smallwhen the average grid potential has a large negative value and also whenit has a positive or only slightly negative value such that the platecurrent is in the level region of the graph and cannot change. ForImoderate negative average grid potentials between these two extremeones .same value of input produces different plate the alternatingcomponent of the plate current takes on its largest values. However, onaccount of 'the feedback arrangement of the circuit some of`thisalternating component ol the plate current will be fed back throughcondenser 5 and rectified by rectiiiers 1 and 8, thereby altering theaverage grid potential. Since the rectiflers areso poled that theirdirect current potential acts to make the grid potential more positivethe tube circuit automatically adjusts itself to av stable pointwhere-the rectified por` input andthe input where the plate current isat tion of the plate current is a maximum and the average grid potentialis still a minus quantity. As the fundamental frequency component of theplate current reaches its maximum at a net average grid potential wherethe graph is steepest and the stronger harmonic components (i. e., theeven order components) have maxima near the top of the hump but alwayson the negative side of the top, the' stable operating point must liebetween the steepest part of the graph and the top of its hump, theexact point depending `upon the distorting characteristics of the tube.This is a narrowly defined region with a strong flow of directcurrent-plate current whereby operation of the relay is effected. Sincea. deviation in either direction of the net average grid potential fromthis operating point'will cause a lowering oflthe rectified componentofthe plate current the result will be a change of a compensating natureso that stability is realized. Thus the resistance of the winding of therelay 4 is of material interest.

yBy means of the variable resistance I3 connected in a network withresistances Il and l5 the input of the circuit may be changed over a.wide range.v By operating this variable resistance I3 and observing thevalues of the plate current the graph of Fig. 6 may be obtained. This isa characteristics curve depicting the relation between the input to thecircuit and the resulting plate current. It should be especially notedthat the curve obtained on decreasing values of input lies alwayssomewhat below thecurve obtained on increasing values of input. Put inanother way, it will be observed that the currents in accordance withthe direction of change in input. Thus for a given value of input theplate current reaches a higher value on increasing input than ondecreasing input. Since a relay requires more current to operate it thanto maintain it operated, this characteristlc of the circuit may beemployed through suitable adjustment of values to cause the relay tooperate and release on exactly the same input.

Fig. 7 shows a family of curves illustrating the effect of the negativebiasing potential of the battery between ground and condenser 6 inFig. 1. Here it is found lthat in all of the graphs except those for thelowest negative biasing potentialsa well-defined rise in plate currentbeing to develop` below the input for which grid current begins to flow.The absence of such a rise indicates a bias insufficiently negative toget the maximum rectified plate current, whereupon no reactive effectsof the feedback circuit can be realized unless the polarity of therectiflers is reversed. This procedure would be undesirable because thegrid could easily go positive and because there would not be enoughchange in plate current to make available safe operating margins for therelay. With the circuit as in Fig. 1 it will be seen from Fig. 7 that atsome point between the highest and the lowest negative biasingpotentials a value maybe chosen where the maximum rise in plate currenttakes place `before the point at which grid current begins to flow.

An examination of the graphs of Fig. 7 shows that the difference inplate current between low a maximum is small for graph 3l, is larger forgraph 32, is largest for graph 33 Aand then progressively becomessmaller for graphs 34, 35 and 36, respectively. Graph 33 shows themaximum change ln plate current between low input and the inputproducing the maximum point of this particular graph. In this figure thegraph 31 indicates the point at which grid -current begins to flow andsince the maximum point in graph 33 is reached before the graph 31 isreached,

stability will be produced without drawing grid current. After thediscovery of these maximum points in the relation between plate currentandI grid potential in one instance and between input and the change ofplate current in the second instance it becomes an engineering problemto select the proper values of the various circuit factors in order todevise a circuit wherein limitation is achieved by biasing potential andload resistance rather than by the harmful flow of grid current. y

In order to further explain the action of this circuit the feedbackcircuit may be uncoupled and the circuit arrangement of Fig. 3 employed.The reverse biasing voltage V (across condenser I6) and theroot-mean-square feedback current I (in conductor Il) may be measured atseveral values of grid bias over a given range of inputs. The result isshown in the family of curves of Fig. 8 wherein the relation between Vand the grid potential is shown, each curve representing a differentvalue of input. 'I'his reverse biasing voltage V results fromrectification of the alternating component of the plate current fed backthrough condenser 5.

Since reconnection of the feedback to the grid circuit requires thatwhere Eg is the grid potential and Ec is the potential of the gridbiasing battery, one of the family of diagonal lines drawn to satisfythis relation defines the state in which the retroactive circuit willbe.l Thus, following the line corresponding to the particular value ofthe fixed grid bias, its intersection with the curve of rectifledvoltage for any given input defines the net average grid potential, Eg,from which the plate current can be ascertained by reference to thestatic characteristics shown in Fig. 8. When a curve is intersectedlmorethan once by the same line, for instance whereline I8 `intersects curveI9, the several values of Eg specify a multi-valued region in thecharacteristic of plate current versus input volume and the actual valuedepends upon the direction of approach and thus explains the phenomenonillustrated by Fig- 6. these values it is necessary to bear in mind theultimateextension of all the curves of rectified voltage asymptoticallytoward zero at sufficiently large negative grid potentials, to avoidoverlooking an intersection beyond the plotted data. The

number of intersections between a line and curve on this graphmustalways be odd, and when plural the two extreme ones specify thestable positions for the two directions of approach.

While it is apparent that these regions of instability may be avoided byreducing thev fixed In finding the relay will remain operated for a timea longer than the incoming pulse. For ncoming pulses of shorter durationthe relay will remain operated for a time b which in practice issubstantially constant. The graph does not extend completely from thezero point as it has been yfound that there is a minimum length ofincoming pulse below which the circuit will not respond but there is agreat range over which the relay grid bias, it is more the object ofthis invention to control and employ the unstable behavior of thiscircuit and to turn such unstable behavior to a useful purpose. Hence byexploring the separation between the two curves of Fig. 6 and thenemploying a relay whose difference between operating and releasingcurrent is equal to the difference in anode currents on rising andfalling inputs, a signal device may be produced which will operate andrelease on exactly the same value of input when changing in eitherdirection. To prolong the interval during which the relay 'remainsoperated, the arrangement of Fig. 4 may be employed. A condenser 20,bridged across the will remain operated a definite length of time inresponse to stimulating pulses of shorter duration. Since the locationof the graph of Fig. 9 may be variously changed` with respect to thelocus shown in dot and dash lines it is possible to produce any givenresult. By way of example.

such a circuit may be employed as a pulse regenerator in a signalingcircuit where equal length signals are used as in some permutation codesignalingv circuits. yIn such a case the circuit may be adjusted so thatthe time interval b will exactly equal the standard length of a pulseemployed in such a circuit.

It has hereinbefore been mentioned that an object of the invention is tomake use of the factors leading to unstable behavior in this 'type ofcircuit. Two kinds of instability are manifest in all the arrangementsof the device described. One kind is the ambiguity of the plate currentwhich depends in its value upon the direction of its change and theother kind consists of oscillations in the circuit, and although theseappear atfirst to be unrelated there is a recognizable connectionbetween the two. As mentioned hereinbefore an-d particularly inconnection with Fig. 6 the decreasing plate current in the circuit ofFig. l or 2 lies below the increasing values. This is controlled bythe'value of the feedback condenser 5. When this `condenser vhas a lowvalue of capacity the values of plate current are as above stated.However,v

' and decreasing values `is reversed-so that now the winding of relay2|, charges during the fore part I of an actuating pulse, and uponcessation of the pulse discharges through the relay to keep itoperatedlonger. Successive pulses separated by less than the hold-overtime will keep the relay operated continuously. The feedback condenser22 may equally well be tapped off the other junction of the relay 2| andits condenser 20 to providii feedback path direct from the plate, inwhich case the condenser 20 then does not form part of the feedbackcircuit.

The graph shown in Fig. 9 indicates the action of the relay 2i and hasbeen drawn from data gathered under controlled experimental conditions.A circuit such as that shown in the lefthand portion of Fig. 4 may beused. A source of alternating current 23 is used `to stimulate thefeedback thermionic tube circuit. The resistances n24 to 21 form partsof attenuators whereby by means of the variable resistances 28 and 29adjustments of volume and potential at the input may be regulated. Bymeans of key 30 pulses of any length may be produced.

The graph of Fig. 9 shows that with an incoming pulse of any durationlonger than the time b input.

values of plate current for decreasing input lie above the values ofplate current for increasing This relationship may be carried so farthat sustained oscillations are set up and the plate current havingattained a given value will remain thereafter at that value regardlessof any further change in input. With a circuit so adjusted .by a propervalue of capacity of the feedback condenser the relay may be made tooperate in the manner of a trigger tube, that is, it may be energized bya short and weak impulse and thereafter remain energized until someother switching function is perfumed to release it.4

What is claimed is:

l. A rectified reaction thermionlc tube circuit comprising a multigridtube, a 'feedback circuit therefor and a condenser in said feedbackcircuit for controlling the relation between the characteristic curve ofanode current with respect to input to said circuit on increasing inputand the said characteristic curve for decreasing in- Put.

2. A rectified reaction thermionic tube circuit n che still other valuesbetween said ranges the said characteristic relation on decreasing inputwill assume a steady and unchanging value.

3. A rectified reaction thermionic tube circuit comprising a multigridtube having a high figure of merit, a relay in the anode circuit thereofand a negative grid biasing battery, the resistance of said relay andthe potential of said battery being chosen within ranges where a maximumin anode current is produced before the grid bias reaches a point wherearid current besins to ow, said relay having a diner-ence in itsoperating and releasing current characteristics equal to the diiiexencein anode current characteristics on rising and fallinginput to saidcircuit.

i. A rectified reaction' thermionic tube circuit comprising a multigridtube, a feedback circuit therefor, a condenser in said feedback circuit,a negative grid biasing battery for controlling the potential, the gridof said tube and a relay in the anode circuit of said tube, theresistance of said relay being chosen to control the characteristicrelation between plate current and grid potential whereby a maximum inplate current is produced before grid current begins to iiow and thevalue of'said negative gridbiasing batrelation between plate current andinput whereby a maximum change in plate current is produced before grid.current begins to ilow.

5. A rectified reaction thermionic tube circuit having a relay in theoutput thereof, said relay having a diiference between its operating andreleasing current characteristics equal to the difference` betweenoutput current characteristics of said circuit on rising and fallinginput thereto.

6. A rectified reaction thermioni tube circuit coxruirising a multigridtube. a feedback circuit thereforand acondenserinsaidfeedbackcircuithaving a capacity adjusted to\oause the graph l tery being chosen tocontrol the characteristic` fcomprlsing a multigrld tube, a feedbackcircuit therefor and a condenservin s aid feedback circuit having acapacity adjusted to cause the anode current to rise to a kgiven valuein response to an input stimulus and to thereafter automaticallymaintain such value regardless of the presence or absence of inputstimulus. A

9. A rectliled reaction thermionic tube circuit comprising a multigridtube, a relay in the anode `circuit thereof .and a condenser bridgedabout said relay for rendering said relay slow to release, said relaycontrolled by said condenser acting to remain operated for a definiteand uni-` form time interval when stimulated by an input impulse of anyfinite duration shorter than said interval. ,r

l0. A rectined reaction thermionic tube circuit having a. relay in theoutputthereof. said relay having a difierence between its operating andreleasing current characteristics and means in said tube circuit foradjusting the difference between the output current characteristics ofsaid circuit on rising and falling input thereto to substantially thesaid diiierence between the over` atilig and releasing currentcharacteristics of said re y. f .Y

ROBERT M. KALB.

